NATs and their operation are in principle known in the art. In principle NATs serve for establishing a session between a user terminal of a private network and a server of a public network, in particular the internet. The establishment is done by allocating a private address and port of the NAT, representing the user terminal to an external address and port representing said server. If a new session between a user terminal and a server is initiated in particular by said user terminal the NAT determines said external IP address and port as well as its own public IP address. Further, for each new session the symmetric NATs in the prior art select a new public NAT-IP port and check if there is an entry of said newly selected public NAT-IP port in a first NAT-table.
Said first NAT-table represents sessions currently allowed to run via said NAT. In the case that there is such an entry of a newly selected public NAT port already included in said NAT-table in the prior art the NATs select another public NAT-IP port for said new session and checks if said other public NAT-IP port is part of an entry of said first NAT-table. These steps of selecting another public NAT-IP port and checking said other NAT-IP port with the entries of said first NAT-table is repeated as long as a public NAT-IP port has been selected which is not already part of an entry in said first NAT-table. In that case a new entry comprising the finally selected public NAT-IP port is input into said first NAT-table. Based on said new entry the NAT is embodied to set up the desired new session between the user terminal and the server of the public network.
As mentioned in the previous paragraph, in the prior art each new session is represented by a new public NAT-IP port. Typically, a public NAT-IP port is identified by a 16 bit word. Consequently, in the prior art the number of sessions which can be distinguished and handled simultaneously by the NAT is limited to a number of 216 sessions. Expressed in other words, the NAT is not able to handle more than 216 sessions simultaneously with the result that each session going beyond that number is not set up by the NAT.
Starting from that prior art it is the object of the invention to improve a known method to operate a NAT as well as a known computer program or a known NAT for carrying out said method such that the session capacity, i.e. the number of sessions which can be distinguished or handled simultaneously by the NAT is increased without the number of bits representing an address and port of the user terminal, the NAT or the external server being increased either.